The invention generally provides for patterned ion-bombarded graphite field emission electron emitters, processes for producing them and their use in field emitter cathode assemblies in flat panel displays.
Field emission electron sources, often referred to as field emission materials or field emitters, can be used in a variety of electronic applications, e.g., vacuum electronic devices, flat panel computer and television displays, emission gate amplifiers and klystrons and in lighting.
Display screens are used in a wide variety of applications such as home and commercial televisions, laptop and desktop computers and indoor and outdoor advertising and information presentations. Flat panel displays are only a few inches thick in contrast to the deep cathode ray tube monitors found on most televisions and desktop computers . Flat panel displays are a necessity for laptop computers, but also provide advantages in weight and size for many of the other applications. Currently laptop computer flat panel displays use liquid crystals which can be switched from a transparent state to an opaque one by the application of small electrical signals. It is difficult to reliably produce these displays in sizes larger than that suitable for laptop computers or for operation over a wide temperature range.
Plasma displays have been used as an alternative to liquid crystal displays. A plasma display uses tiny pixel cells of electrically charged gases to produce an image and requires relatively high electrical power to operate.
Flat panel displays having a cathode assembly using a field emission electron source, i.e., a field emission material or field emitter, and a phosphor capable of emitting light upon bombardment by electrons emitted by the field emitter have been proposed. Such displays have the potential for providing the visual display advantages of the conventional cathode ray tube and the depth and weight advantages of the other flat panel displays with the additional advantage of lower power consumption than the other flat panel displays. U.S. Pat. Nos. 4,857,799 and 5,015,912 disclose matrix-addressed flat panel displays using micro-tip cathodes constructed of tungsten, molybdenum or silicon. WO 94-15352, WO 94-15350 and WO 94-28571 disclose flat panel displays wherein the cathodes have relatively flat emission surfaces.
Field emission has been observed in two kinds of nanotube carbon structures. L. A. Chernozatonskii et al., Chem. Phys. Letters 233, 63 (1995) and Mat. Res. Soc. Symp. Proc. Vol. 359, 99 (1995) have produced films of nanotube carbon structures on various substrates by the electron evaporation of graphite in 10xe2x88x925-10xe2x88x926 torr. These films consist of aligned tube-like carbon molecules standing next to one another. Two types of tube-like molecules are formed; the A-tubelites whose structure includes single-layer graphite-like tubules forming filaments-bundles 10-30 nm in diameter and the B-tubelites, including mostly multilayer graphite-like tubes 10-30 nm in diameter with conoid or dome-like caps. They report considerable field electron emission from the surface of these structures and attribute it to the high concentration of the field at the nanodimensional tips. B. H. Fishbine et al., Mat. Res. Soc. Symp. Proc. Vol. 359, 93 (1995) discuss experiments and theory directed towards the development of a buckytube (i.e., a carbon nanotube) cold field emitter array cathode.
R. S. Robinson et al ., J. Vac. Sci. Technolo. 21, 1398 (1983) disclose the formation of cones on the surfaces of substrates under ion bombardment. The effect was reported for various substrate materials and were generated by simultaneously sputtering a surface at high energy while seeding it with impurity atoms deposited at low energy. They also disclosed the formation of carbon whiskers up to 50 xcexcm in length when a graphite substrate was ion-bombarded with impurities from a stainless steel target.
J. A. Floro et al., J. Vac. Sci. Technolo. A 1, 1398 (1983) disclose the formation of whiskers during relatively high current density ion bombardment of heated graphite substrates. The whiskers were disclosed to be 2-50 xcexcm in length and 0.05-0.5 xcexcm in diameter and to grow parallel to the ion beam. Simultaneous impurity seeding was reported to inhibit whisker growth. J. A. van Vechten et al., J. Crystal Growth 82, 289 (1987) discuss the growth of whiskers from graphite surfaces under ion sputtering conditions. They note that the whiskers of smallest diameter, characteristically about 15 nm, definitely appear to be different from either diamond or the scrolled-graphite structure found in carbon fibers grown by catalytic pyrolysis of hydrocarbons. Larger whiskers with diameters ranging from 30 to 100 nm were also observed to grow in sputtering systems. The smaller diameter whiskers are constant in diameter along the length while the larger diameter whiskers may have a slight taper.
M. S. Dresselhaus et al., Graphite Fibers and Filaments (Springer-Verlag, Berlin, 1988), pp. 32-34, disclose that filaments may be grown on several types of hexagonal carbon surfaces, but not on diamond or glassy carbon.
T. Asano et al., J. Vac. Sci. Technol. B 13, 431 (1995) disclose increased electron emission from diamond films which have been deposited on silicon by chemical vapor deposition, argon ion milled to form diamond cones and then annealed at 600xc2x0 C. These cones are formed if the diamond is in the form of isolated grains.
C. Nxc3xctzenadel et al., Appl. Phys. Lett. 69, 2662 (1996) disclose field emission from cones etched into both synthetic boron-doped diamond and silicon by ion sputtering.
S. Bajic et al., J. Phys. D: Appl. Phys. 21, 200 (1988) disclose a field emitter composite with graphite particles suspended in a resin layer.
R. A. Tuck et al., WO 97/06549, disclose a field emission material comprising an electrically conductive substrate and, disposed thereon, electrically conductive particles embedded in, formed in, or coated by a layer of inorganic electrically insulating material to define a first thickness of the insulating material between the particle and the substrate and a second thickness of the insulating material between the particle and the environment. The field emitting material may be printed onto a substrate.
M. Rabinowitz, U.S. Pat. No. 5,697,827, discloses a method and apparatus for producing, maintaining and generating a cathode source of thermo-field assisted emission of electrons and regeneration of the electric field enhancing whisker component of this source. The only carbon whiskers disclosed were carbon nanotubes. To form the cathode these nanotubes are bonded to a support by propelling them by an electric field and thereby embedding them into a soft material shell surrounding the support.
Despite the prior art, there is a need for a process for readily and economically producing both small and large sized highly emitting field emission electron emitters for use in various flat panel applications. Other objects and advantages of the present invention will become apparent to those skilled in the art upon reference to the drawings and detailed description which follow hereinafter.
The invention provides a process for producing a field emission electron emitter, which comprises the steps of:
(a) forming a layer of composite which comprises graphite particles and glass on a substrate, wherein the glass adheres to the substrate and to portions of the graphite particles thereby affixing the graphite particles to one another and to the substrate and wherein at least 50% of the surface area of the layer of composite consists of portions of graphite particles, and
(b) bombarding the surface of the layer formed in (a) with an ion beam which comprises ions of argon, neon, krypton or xenon for a time sufficient to form whiskers on said graphite particles.
Preferably, at least 70% of the surface area of the layer of composite consists of portions of graphite particles.
The volume per cent of graphite particles is about 35% to about 80% of the total volume of the graphite particles and the glass, preferably about 50% to about 80% of the total volume.
The invention also provides a process for producing a field emission electron emitter wherein the composite further comprises electrically conducting material.
Preferably, the ion beam is an argon ion beam and the argon ion beam has an ion current density of from about 0.1 mA/cm2 to about 1.5 mA/cm2, a beam energy of from about 0.5 keV to about 2.5 keg and the period of ion bombardment is about 15 minutes to about 90 minutes. More preferred are ion beam gas compositions comprising argon and nitrogen.
Preferably, the glass is a low softening point glass.
Preferably, the electrically conducting material is silver or gold.
Preferably, when the layer of composite comprises graphite and glass, the process for forming the layer of composite on a substrate comprises screen printing a paste comprised of graphite particles and glass frit onto the substrate in the desired pattern and firing the dried patterned paste. For a wider variety of applications, e.g., those requiring finer resolution, the preferred process comprises screen printing a paste which further comprises a photoinitiator and a photohardenable monomer, photopatterning the dried paste and firing the patterned dried paste.
Preferably, when the layer of composite further comprises an electrically conducting material, the process for forming the layer of composite on a substrate comprises screen printing a paste comprised of graphite, glass frit and an electrically conducting material onto the substrate in the desired pattern and firing the dried patterned paste. For a wider variety of applications, e.g., those requiring finer resolution, the preferred process comprises screen printing a paste which further comprises a photoinitiator and a photohardenable monomer, photopatterning the dried paste and firing the patterned dried paste.
Preferably, when the substrate is glass, the dried patterned paste is fired at a temperature of about 450xc2x0 C. to about 575xc2x0 C., most preferably at about 525xc2x0 C., for about 10 minutes. Preferably, the thickness of the fired layer of composite is from about 5 xcexcm to about 30 xcexcm.
The invention also provides a screen printable or coatable paste that can be used in the preferred process for forming a layer of composite comprising graphite and glass on a substrate. Based on the total weight of the paste, the paste contains about 40 wt % to about 60 wt % solids. The solids are comprised of graphite particles and glass frit or graphite, glass frit and an electrically conducting material. The volume per cent of graphite particles is about 35% to about 80% of the total volume of solids, preferably about 50% to about 80% of the total volume. The graphite particle size is preferably about 0.5 xcexcm to about 10 xcexcm.
In addition, the invention provides a process for forming a layer of composite which comprises graphite and glass on a substrate, which comprises:
(a) screen printing a paste comprised of graphite particles and glass frit onto the substrate in a desired pattern, wherein the volume per cent of graphite particles is about 35% to about 80% of the total volume of the graphite particles and the glass frit, and
(b) firing the dried patterned paste to soften the glass frit and cause it to adhere to the substrate and to portions of the graphite particles thereby affixing the graphite particles to one another and to the substrate to produce the layer of composite, wherein at least 50% of the surface area of the layer of composite consists of portions of graphite particles.
Preferably, at least 70% of the surface area of the layer of composite consists of portions of graphite particles.
The invention also provides a process for forming a layer of composite which comprises graphite and glass on a substrate, which comprises:
(a) screen printing a paste comprised of graphite particles, glass frit, a photoinitiator and a photohardenable monomer onto the substrate, wherein the volume per cent of graphite particles is about 35% to about 80% of the total volume of the graphite particles and the glass frit,
(b) photopatterning the dried paste, and
(c) firing the patterned dried paste to soften the glass frit and cause it to adhere to the substrate and to portions of the graphite particles thereby affixing the graphite particles to one another and to the substrate to produce the layer of composite, wherein at least 50% of the surface area of the layer of composite consists of portions of graphite particles.
Preferably, at least 70% of the surface area of the layer of composite consists of portions of graphite particles.
In addition, the invention provides for a layer of composite which comprises graphite and glass on a substrate made by the above process and which can be subsequently treated to produce a field emission electron emitter. In the layer of composite which comprises graphite and glass, the volume per cent of graphite particles is about 35% to about 80% of the total volume of the graphite particles and the glass, preferably about 50% to about 80% of the total volume.
The invention also provides electron emitters produced by the process of this invention. These electron emitters and field emitter cathode assemblies made therefrom are useful in vacuum electronic devices, flat panel computer and television displays, emission gate amplifiers, klystrons and lighting devices. The panel displays can be planar or curved.
The panel displays provided by the present invention comprise a cathode assembly comprised of an electron emitter produced by the process of this invention, an anode spaced apart from the cathode assembly, the anode including a layer of a patterned optically transparent conductive film upon a cathode-facing surface of an anode support plate, and a layer of a phosphor capable of emitting light upon bombardment by electrons emitted by the electron emitter of the cathode assembly, the phosphor layer situated adjacent the layer of patterned optically transparent conductive film between the anode and cathode, and a gate electrode situated between the anode and the cathode, the gate electrode including a structure of conductive paths arranged substantially orthogonal to the patterned optically transparent conductive film, each conductive path selectively operably connected to an electron source, and a voltage source connected between the anode and electron emitter. In addition to the single gate electrode in the above-described triode structure, additional controlling electrodes can be used; their use can allow lower emission voltages on the gate electrode and provide higher acceleration voltages. These additional electrodes also provide a means to adjust the field pattern and the emission and to focus the emitted electrons.